There are actually a lot of jobs that military members can't perform. The civilians are there as continuity. Military active duty personnel move around too much. Civilians typically stay at one location their whole careers. At a lot of locations the civilians are the ones that do the training and keep the mission flowing smoothly.
It sounds good to say cut civilians but that is really not such a cost effective idea. A lot of the depot work was always done by civilians. You would have to create new career fields, train and recruit to replace the civilians in these slots. This would take years to accomplish. Not really a savings.
Milpers don't HAVE to move around so much. The civilians have about as much effect on the morale of the qualified uniforms as do homosexuals having been placed atop a pedestal as some honored species.